Summary of previous lesson Macrophage Attacking E.coli (SEM x8,800) © Dr Dennis KunkelDr Dennis Kunkel Alveolar (Lung) Macrophage Attacking E. coli (SEM x10,000) © Dr Dennis Kunkel (used with permission)Dr Dennis Kunkel

Horton & Parker: Informed Infection Control Practice I. Chain of Infection

HOST DEFENSE MECHANISMS

Lecture nr. 6 Antigens. Antibodies. Immune response. Cell cooperation and mediators in immune response

HOST DEFENSE MECHANISMS

Immunity: Free from burden. Ability of an organism to recognize and defend itself against specific pathogens or antigens. Immunity refers to the ability of the human body to resist disease agents and their toxins through possession of antibodies. Immunity is a long lasting protection against a specific antigen. Definitions of the Immunity

Specific Host Defenses: The Immune Response The structure of immunoglobulins T-cell (part of the specific immune response)

The Immune Response Immune Response: Third line of defense. Involves production of antibodies and generation of specialized lymphocytes against specific antigens.

What is the immune system? The bodys defense against disease causing organisms, malfunctioning cells, and foreign particles

Microbes are recognized by two mechanisms: evolved broad recognition mechanisms (innate immunity), and by highly specific lymphocyte antibodies and T cell receptors (adaptive immunity) Different types of microbes are eliminated by different effectors mechanisms, which are designed to best combat each type of microbe How does the immune system respond to different infections?

Classification of immunity

Characteristics of Innate and Adaptive Immunity No Immunologic memory Antigen independent Immediate Genetically determined Antigen dependent Take time Not genetically determined. Development of memory Innate Immunity Acquired Immunity

Classification of the immunity

A. Naturally Acquired Active Immunity: Antigens or pathogens enter body naturally. Body generates an immune response to antigens. Immunity may be lifelong (chickenpox or mumps) or temporary (influenza or intestinal infections). B. Naturally Acquired Passive Immunity: Antibodies pass from mother to fetus via placenta or breast feeding (colostrum). No immune response to antigens. Immunity is usually short-lived (weeks to months). Protection until childs immune system develops. Acquired Immunity: Obtained in the course of daily life.

Classification of the immunity

1. Artificially Acquired Active Immunity: Antigens are introduced in vaccines (immunization). Body generates an immune response to antigens. Immunity can be lifelong (oral polio vaccine) or temporary (tetanus toxoid). 2. Artificially Acquired Passive Immunity: Preformed antibodies (antiserum) are introduced into body by injection. Snake antivenom injection from horses or rabbits. Immunity is short lived (half life three weeks). Host immune system does not respond to antigens. II. Artificially Acquired Immunity: Obtained by receiving a vaccine or immune serum.

u Beneficial: u Protection from Invaders u Elimination of Altered Self u Detrimental: u Discomfort (inflammation) u Damage to self (autoimmunity) u Beneficial: u Protection from Invaders u Elimination of Altered Self u Detrimental: u Discomfort (inflammation) u Damage to self (autoimmunity) Significance of the Immune System

Components of Human Immune System

Cells of the immune system Lymphocytes Mediators of adaptive immune responses; only cells with specific receptors for antigens Antigen-presenting cells (APCs) Specialized to capture, concentrate, and display antigens for recognition by lymphocytes Dendritic cells; macrophages, B cells; follicular dendritic cells Different APCs serve different roles in adaptive immune responses Effector cells Function to eliminate microbes; include lymphocytes, granulocytes (neutrophils, eosinophils), macrophages

Lymphocytes Originate : in liver, spleen and bone marrow of fetus in bone marrow after birth From stem cells – hemocytoblasts – that produce all blood cells.

To become mature, immunocompetent cells, they must pass through lymphoid tissues in other parts of the body. As they do so, they become committed to becoming either T cells or B cells 1. Cells that migrate through the bone marrow (bursal equivalent) become B cells, and will produce antibodies and participate in humoral immunity. 2. Cells that migrate through the thymus glands become T cells and participate in Cell-mediated immunity. Lymphocytes

Congenital immunodeficiency diseases are often caused by blocks at different stages of lymphocyte maturation LYMPHOCYTE DEVELOPMENT

CD Nomenclature Structurally defined leukocyte surface molecule that is expressed on cells of a particular lineage (differentiation) and recognized by a group (cluster) of monoclonal antibodies is called a member of a cluster of differentiation (CD) CD molecules (CD antigens, CD markers) are: Identified by numbers Used to classify leukocytes into functionally distinct subpopulations, e.g. helper T cells are CD4+CD8-, CTLs are CD8+CD4- Often involved in leukocyte functions Antibodies against various CD molecules are used to: Identify and isolate leukocyte subpopulations Study functions of leukocytes Eliminate particular cell populations

Two types of T cells CD4 T cells Help other immune cells Recognize peptide + MHC II MHC II is expressed primarily on immune cells Peptides are from endocytosed antigen CD8 T cells Kill virus-infected cells Recognize peptide + MHC I MHC I is expressed on all nucleated cells Peptides are from cytosolic antigen

ANTIGENS

An antigen is a molecule that stimulates an immune response. The word originated from the notion that they can stimulate antibody generation. The modern definition includes all substances that can be recognized by the adaptive immune system. Antigens

Antigens - substances recognized as non-self. These can be: Infectious agents - bacteria, viruses, fungi or parasites Noninfectious substances – Environmental - pollen, foods, bee, venoms Drugs, vaccines, transfusions and transplanted tissues Antigens

Chemical nature of antigen Proteins Polysaccharides Nucleic acids Lipids Some glycolipids and phospholipids can be immunogenic for T cells and illicit a cell-mediated immune response

uMost are proteins or large polysaccharides from a foreign organism. Microbes: Capsules, cell walls, toxins, viral capsids, flagella, etc. Nonmicrobes: Pollen, egg white, red blood cell surface molecules, serum proteins, and surface molecules from transplanted tissue. uLipids and nucleic acids are only antigenic when combined with proteins or polysaccharides. Antigens

Antibody Generator The best antigens are: 1. large 2. recognized as foreign 3. complex

Antigens can be classified in order of their origins Exogenous antigens - are antigens that have entered the body from the outside, ex. by inhalation, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. Endogenous antigens - antigens that have been generated within the cell, as a result of: normal cell metabolism, or viral or intracellular bacterial infection.

Allogenic antigen The specific antigen exists in different individuals. Blood type antigens. Autoantigens usually a normal protein (sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease. Types of Antigens

are presented by the MHC I molecules on the surface of tumor cells. can sometimes be presented only by tumor cells and never by the normal cells. they are called tumor-specific antigens (TSAs) and typically result from a tumor specific mutation. Tumor antigens

Characteristics of Antigen Immunogenicity The capacity to stimulate the production of antibodies or cell-mediated immune responses.

Antigenicity The ability to bind antibody. Complete antigen Incomplete antigen, also known as hapten. Hapten: Small foreign molecule that is not antigenic. Must be coupled to a carrier molecule to be antigenic. Once antibodies are formed they will recognize hapten. Characteristics of Antigen

Epitope or, Antigenic determinants: uSmall part of an antigen that interacts with an antibody. uAny given antigen may have several epitopes. uEach epitope is recognized by a different antibody and may interact with them by paratopes of Antibody Antigens FeFe

Epitopes: Antigen Regions that Interact with paratopes, Antibodies regions

Types of Epitopes 1. Linear epitopes continuous and found in polysaccharides as well as in both native (nondenatured) and denatured proteins, especially fibrillar proteins. specificity depends upon primary sequence. typical size is 5-6 subunits in length.

2. Conformational epitopes Discontinuous involve multiple subunits, often located far apart in the primary sequence of the antigen molecule and are thus found only in native (globular) proteins. Types of Epitopes

Antigenic epitopes

ANTIBODY

uProteins that recognize and bind to a particular antigen with very high specificity. uMade in response to exposure to the antigen. uOne virus or microbe may have several antigenic determinant sites, to which different antibodies may bind. uEach antibody has at least two identical sites that bind antigen: antigen binding sites. uValence of an antibody: Number of antigen binding sites. Most are bivalent. uBelong to a group of serum proteins called immunoglobulins (Igs). Antibodies

uMonomer: A flexible Y-shaped molecule with four protein chains: 2 identical light chains 2 identical heavy chains uVariable Regions: Two sections at the end of Ys arms. Contain the antigen binding sites (Fab). Identical on the same antibody, but vary from one antibody to another. uConstant Regions: Stem of monomer and lower parts of Y arms. uFc region: Stem of monomer only. Important because they can bind to complement or cells. Antibody Structure

Immunoglobulins IgG - monomer IgA – dimer – 2 units IgM – pentamer – 5 units IgD – monomer IgE – monomer

uStructure: Monomer uPercentage serum antibodies: 80% uLocation: Blood, lymph, intestine uHalf-life in serum: 23 days uComplement Fixation: Yes uPlacental Transfer: Yes uKnown Functions: Enhances phagocytosis, neutralizes toxins and viruses, protects fetus and newborn. I. IgG

uStructure: Pentamer uPercentage serum antibodies: 5-10% uLocation: Blood, lymph, B cell surface (monomer) uHalf-life in serum: 5 days uComplement Fixation: Yes uPlacental Transfer: No uKnown Functions: First antibodies produced during an infection. Effective against microbes and agglutinating antigens. II. IgM

uStructure: Dimer uPercentage serum antibodies: 10-15% uLocation: Secretions (tears, saliva, intestine, milk), blood and lymph. uHalf-life in serum: 6 days uComplement Fixation: No uPlacental Transfer: No uKnown Functions: Localized protection of mucosal surfaces. Provides immunity to infant digestive tract. III. IgA

uStructure: Monomer uPercentage serum antibodies: 0.2% uLocation: B-cell surface, blood, and lymph uHalf-life in serum: 3 days uComplement Fixation: No uPlacental Transfer: No uKnown Functions: In serum function is unknown. On B cell surface, initiate immune response. IV. IgD

uStructure: Monomer uPercentage serum antibodies: 0.002% uLocation: Bound to mast cells and basophils throughout body. Blood. uHalf-life in serum: 2 days uComplement Fixation: No uPlacental Transfer: No uKnown Functions: Allergic reactions. Possibly lysis of worms. V. IgE

Clonal Selection of B Cells is Caused by Antigenic Stimulation

B cells develop from stem cells in the bone marrow of adults (liver of fetuses). After maturation B cells migrate to lymphoid organs (lymph node or spleen). Clonal Selection: When a B cell encounters an antigen it recognizes, it is stimulated and divides into many clones called plasma cells, which actively secrete antibodies. Each B cell produces antibodies that will recognize only one antigenic determinant. Secretions the Antibodies by B cells

Programmed cell death (Falling away). Human body makes 100 million lymphocytes every day. If an equivalent number doesnt die, will develop leukemia. B cells that do not encounter stimulating antigen will self-destruct and send signals to phagocytes to dispose of their remains. Many virus infected cells will undergo apoptosis, to help prevent spread of the infection. Apoptosis

Antigen presenting cells

Cell surface peptides of Ag Antigens must be processed in order to be recognised by T cells Y T T cell response No T cell response No T cell response No T cell response No T cell response Soluble native Ag Cell surface native Ag Soluble peptides of Ag Cell surface peptides of Ag presented by cells that express MHC antigens ANTIGEN PROCESSING

M M M M 0mins60mins T Specific T cells The interaction of T cells with macrophages requires antigen catabolism Microbes NO T CELLS BIND T CELLS BIND NO T CELLS BIND NO T CELLS BIND T cell do not bind stably to antigen presenting cells unless the antigen is catabolised

T and B cells recognise antigen differently Antigen must be catabolised before T cells can recognise it Antigen processing generates antigenic peptides Exogenous antigen processing takes place in lysosomes Endogenous processing is non-lysosomal The mechanism of antigen processing depends upon the compartment in which the pathogen replicates Endogenous and exogenous antigen processing both involve uptake, degradation, complex formation and presentation Pathogens can evade immunity by disrupting antigen processing Summary

Immune response

Duality of Immune System I.Humoral (Antibody- Mediated) Immunity I.II. Cellular (Cell Mediated) Immunity

Involves production of antibodies against foreign antigens. Antibodies are produced by a subset of lymphocytes called B cells. B cells that are stimulated will actively secrete antibodies and are called plasma cells. Antibodies are found in extracellular fluids (blood plasma, lymph, mucus, etc.) and the surface of B cells. Defense against bacteria, bacterial toxins, and viruses that circulate freely in body fluids, before they enter cells. Also cause certain reactions against transplanted tissue. I. Humoral (Antibody- Mediated) Immunity

How do antibodies actually help eliminate antigens?

Antigen-Antibody Complex: Formed when an Ab binds to Ag. Affinity: A measure of binding strength. 1. Agglutination: Antibodies cause antigens (microbes) to clump together. IgM is more effective that IgG. Hemagglutination: Agglutination of red blood cells. Used to determine ABO blood types and to detect influenza and measles viruses. Consequences of Antigen-Antibody Binding

2. Opsonization: Antigen (microbe) is covered with antibodies that enhances its ingestion and lysis by phagocytic cells. 3. Neutralization: IgG inactivates viruses by binding to their surface and neutralize toxins by blocking their active sites.

4. Antibody-dependent cell-mediated cytotoxicity: Used to destroy large organisms (e.g.: worms). Target organism is coated with antibodies and bombarded with chemicals from nonspecific immune cells. 5. Complement Activation: Both IgG and IgM trigger the complement system which results in cell lysis and inflammation. Consequences of Antigen-Antibody Binding

Consequences of Antibody Binding

Primary Response: After initial exposure to antigen, no antibodies are found in serum for several days. A gradual increase in titer, first of IgM and then of IgG is observed. Most B cells become plasma cells, but some B cells become long living memory cells. Gradual decline of antibodies. Immunological responce

Secondary Response: Subsequent exposure to the same antigen displays a faster and more intense antibody response. Increased antibody response is due to the existence of memory cells, which rapidly produce plasma cells upon antigen stimulation. Antibody Titer: The amount of antibody in the serum. Immunological responce

Antibody Response After Exposure to Antigen

Duality of Immune System I.Humoral (Antibody- Mediated) Immunity I.II. Cellular (Cell Mediated) Immunity

Involves specialized set of lymphocytes called T cells that recognize foreign antigens on the surface of cells, organisms, or tissues: Helper T cells Cytotoxic T cells T cells regulate proliferation and activity of other cells of the immune system: B cells, macrophages, neutrophils, etc. Defense against: Bacteria and viruses that are inside host cells and are inaccessible to antibodies. Fungi, protozoa, and helminths Cancer cells Transplanted tissue II. Cellular (Cell Mediated) Immunity

Antigens that stimulate this response are mainly intracellular. Requires constant presence of antigen to remain effective. Unlike humoral immunity, cell mediated immunity is not transferred to the fetus. Cytokines: Chemical messengers of immune cells. Over 100 have been identified. Stimulate and/or regulate immune responses. Interleukins: Communication between WBCs. Interferons: Protect against viral infections. Chemokines: Attract WBCs to infected areas. T Cells and Cell Mediated Immunity

T cells are key cellular component of immunity. T cells have an antigen receptor that recognizes and reacts to a specific antigen (T cell receptor). T cell receptor only recognize antigens combined with major histocompatability (MHC) proteins on the surface of cells. MHC Class I: Found on all cells. MHC Class II: Found on phagocytes. T Cells and Cell Mediated Immunity

T Cells Only Recognize Antigen Associated with MHC Molecules on Cell Surfaces

T ypes of T cells 1. T Helper (T H ) Cells: Central role in immune response. Most are CD4 + Recognize antigen on the surface of antigen presenting cells (e.g.: macrophage). Activate macrophages Induce formation of cytotoxic T cells Stimulate B cells to produce antibodies. Cells and Cell Mediated Immunity

Central Role of Helper T Cells

2. Cytotoxic T (Tc) Cells: Destroy target cells. Most are CD4 negative (CD4 - ). Recognize antigens on the surface of all cells: Kill host cells that are infected with viruses or bacteria. Recognize and kill cancer cells. Recognize and destroy transplanted tissue. Release protein called perforin which forms a pore in target cell, causing lysis of infected cells. Undergo apoptosis when stimulating antigen is gone. Types of T cells (Continued)

Cytotoxic T Cells Lyse Infected Cells

3.Delayed Hypersensitivity T (T D ) Cells: Mostly T helper and a few cytotoxic T cells that are involved in some allergic reactions (poison) and rejection of transplanted tissue. 4. T Suppressor (Ts) Cells: May shut down immune response. Types of T cells (Continued)

1. Activated Macrophages: Stimulated phagocytes. Stimulated by ingestion of antigen Larger and more effective phagocytes. Enhanced ability to eliminate intracellular bacteria, virus-infected and cancerous cells. 2. Natural Killer (NK) Cells: Lymphocytes that destroy virus infected and tumor cells. Not specific. Dont require antigen stimulation. Not phagocytic, but must contact cell in order to lyse it. Nonspecific Cellular Components

Matures inbone marrowthymus Type of immunityhumoralcell-mediated Secretesantibodiescytokines Antigen receptorsurface IgT cell receptor Where foundspleenblood, lymph nodes Targetsbacteria, infected cells, virusestumor cells? MemoryYesYes B and T cells mediate specific immunity B cellsT cells

Cellular Interactions in the Immune Response

Antigen-Presenting cells (macrophages) place antigen on their cell surface in combination with the MHC II complex Ag is presented to a specific helper T cell that has receptors that bind the Ag – MHC II complex

After binding, the APC produces Interleukin -1 (IL-1) which stimulates the T H Cell to produce IL-2 and/or IL-4 Interleukin-2 has an autocrine function, causes T H Cell to clone itself, and make more IL-2 and /or IL-4

Central Role of Helper T Cells

Helper T cells T H1 cells produce IL -2 and IFN- ɣ and influence cell-mediated immunity T H2 cells produce IL -4 (and other ILs) and influence antibody-mediated (humoral) immunity

When B cell comes in contact with the antigen and IL-4, the B cell produces plasma cells and memory cells Tc Cells come in contact with the antigen on the surface of infected cells in combination with the MHC 1 complex. When also have binding with IL-2, cells produce activated Tc Cells and memory cells.

Antibody Production T-Dependent Antigens: Antibody production requires assistance from T helper cells. A macrophage cells ingest antigen and presents it to T H cell. T H cell stimulates B cells specific for antigen to become plasma cells. Antigens are mainly proteins on viruses, bacteria, foreign red blood cells, and hapten-carrier molecules. T-Independent Antigens: Antibody production does not require assistance from T cells. Antigens are mainly polysaccharides or lipopolysaccharides with repeating subunits (bacterial capsules). Weaker immune response than for T-dependent antigens. Relationship Between Cell- Mediated and Humoral Immunity

Humoral Response to T Dependent Antigens

Overview of the Immune Response

Principal mechanisms of defense against microbes Antibodies PhagocytesT cells (CTLs) ( may work with antibodies, T cells) All microbes Intracellular microbes, esp. viruses

Disorders of Immunity

Hypersensitivity Exaggerated Immune Response

Hypersensitivity Types Allergy Exogenous, non-human antigen Isoimmunity (alloimmunity) Exogenous, human antigen Autoimmunity Endogenous antigen

Hypersensitivity Mechanisms Type I: IgE mediated Type II: Tissue specific Type III: Immune complex mediated Type IV: Cell mediated

Type I Immediate hypersensitivity IgE mediated Exogenous antigen Most (but not all) Allergies

Type I: Mechanism Repeated antigen exposure causes increased IgE production IgE binds to mast cells Sensitization occurs

Type I: Mechanism Antigen binds to IgE on mast cell membrane Mast cell releases histamine, chemotaxic factors Inflammatory response occurs

Type I: Signs/Symptoms Clinical signs, symptoms = response to histamine release GI, skin, respiratory system High mast cells numbers Most sensitive

Type I: Signs/Symptoms Histamine effects Vasodilatation Increased capillary permeability Non-vascular smooth muscle spasm

Type I: Signs/Symptoms Skin: flushing, itching, edema, urticaria Respiratory: bronchospasm, laryngospasm, laryngeal edema Cardiovascular: tachycardia, hypotension GI: nausea, vomiting, cramping, diarrhea

Type I: Atopia Allergy prone individuals Genetic predisposition More IgE More mast cell receptors for antibodies than normal

Type I: Anaphylaxis Severe, generalized Type I reaction Life-threatening Loss of airway Ventilatory failure Hypoperfusion

Type II Tissue specific Reaction to tissue-specific antigens Causes target cell destruction, dysfunction Exogenous or endogenous antigen

Type II Most commonly affected cells Red blood cells Thyroid cells

Type II: Mechanisms Antibody binds to cell membrane, triggers compliment-mediated lysis Examples Reaction to transfused blood Hemolytic disease of newborn

Type II: Mechanisms Antibodies promote target cell clearance by macrophages

Type II: Mechanisms Antibodies bind to target cells and cytotoxic T-cells Trigger release of toxins to destroy target cells

Type II: Mechanisms Antibody binds to cell membrane, causes alterations in target cell function Example: Graves' disease Antibody binds to thyroid cell membrane Mimics Thyroid Stimulating Hormone action Causes production of excessive amounts of thyroid hormone Results in common form of hyperthyroidism

Type III Mediated by antigen/ antibody complex deposition in tissues Exogenous or endogenous antigen

Type III: Mechanism Ag-Ab complex deposited in tissues Especially sensitive tissues are blood vessels, GI, respiratory system Causes complement activation, increased neutrophil activity Neutrophils have trouble digesting complexes, release lysosomes causing damage

Type III Immune complex quantity varies over time Symptomatic periods alternate with periods of remission

Type III: Serum Sickness Repeated intravenous antigen injections Immune complexes deposited in tissues Fever, rash, pain, lymphadenopathy

Type III: Raynauds Phenomenon Temperature governs immune complex deposition in peripheral circulation Exposure to cold causes redness, pain of fingers, toes followed by numbness, cyanosis, gangrene

Type III: Arthus Reaction Occurs after repeated LOCAL exposure to exogenous antigen Immune complexes in vessel walls Examples Celiac disease from wheat protein Hemorrhagic alveolitis from moldy hay inhalation

Type IV Delayed Mediated by Td (lymphokine-producing) or Tc (cytotoxic) cells No antibody involved

Type IV Examples Graft rejection Contact allergic reactions (poison ivy)

Hypersensitivity Targets Allergins Pollen (hay fever) Drug reactions Foods

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